A negative glow discharge lamp typically comprises a light-transmitting envelope containing a noble gas and mercury with a phosphor coating on an inner surface of the envelope which is adapted to emit visible light upon absorption of ultraviolet radiation that occurs when the lamp is excited. The lamp is excited by means of the application of a voltage between the lamp electrodes. Current flows between the electrodes after a certain potential is applied to the electrodes, commonly referred to as the breakdown voltage. An elementary explanation of the phenomenon is that the gas between the electrodes becomes ionized at a certain voltage, conducts current, and emits ultraviolet radiation. The phosphor coating on the inner surface of the lamp envelope is caused to fluoresce and re-emit a substantial portion of the ultraviolet radiation as visible light. The spectral characteristics of the visible light is determined principally by the composition of the fluorescent powders used for the phosphor coating. During operation, negative glow discharge lamps generally require a series-connected current-limiting device. Without the current being limited, the discharge potential drops and the current increases until the lamp fails due to current overload.
Prior U.S. patents describe the use various of ballast circuits to control the current. The use of capacitive ballasts with or without rectifier circuits are described in
U.S. Pat. No. 2,356,369 to Abernathy; U.S. Pat. No. 4,288,725 to Morton, U.S. Pat. No. 4,172,981 to Smith, U.S. Pat. No. 4,500,812 to Roche, and U.S. Pat. No. 3,787,751 to Farrow. U.S. Pat. No. 4,952,844 to Godyak et.al. describes a ballast using a rectifier bridge intercoupling a capacitor and the electrodes of the lamp.
One major problem common to negative glow lamps is their sensitivity to operating conditions. The lamp voltage is determined primarily by cathode fall which is in turn dependent on mercury vapor pressure, cathode condition, and amount of external cathode heat provided. When mercury vapor pressure is low, as in the case during lamp warm-up, under dimming, and with low ambient temperature, cathode fall increases so that severe sputtering damage and lowered efficacy due to buffer gas excitation may result. When mercury vapor pressure is high due to high ambient temperature or before stabilization, cathode fall and lamp voltage are low reducing lamp wattage and efficacy. The negative glow lamp is also very sensitive to changes in cathode quality which increase or decrease emissivity resulting in the same problems.
Because of the high discharge current and low voltage attendant with a negative glow discharge lamp, the most practical ballast for the lamp is electronic. Under normal operating conditions, the direct current negative glow lamp requires circulatory current to achieve adequate cathode temperature. In the direct current lamp, the cathode receives its power from a combination of ion bombardment, I.sup.2 R heating from the discharge current, and from the externally supplied circulatory current. These requirements change when the lamp is dimmed or operated in hot or cold environments. Thus, optimization of the ballast circuit to take into account these variables is difficult.
When overheated, the thermionic emission increases and the cathode fall drops below optimum reducing lamp power and efficacy. At some point the discharge becomes unstable and flicker is objectionable. When underheated, the cathode fall increases causing rapidly increasing sputtering damage and shortened life. Therefore, the negative glow lamp is extremely sensitive to operating conditions which shift the lamp voltage away from the optimum range with resulting short life or poor efficacy.